Simply put, ISO is the measurement of a camera’s sensitivity to light. The higher the ISO number the more sensitive the sensor is. A higher ISO number means that a camera will be able to shoot images in conditions with less light. This is one of the many camera terms carried over from the film days–you used to need to change your film in order to change your sensitivity, now you just need to change a setting.

As we discussed in our section on Sensors, a digital camera’s ISO can be a useful gauge of its sensor size, and therefore, its image quality. But taking a strict “more ISO equals more image quality” standpoint is fraught with peril.

First thing’s first: your digital camera’s ISO settings determine how sensitive the sensor is to light. In low light environments, the higher your ISO, the more light you’ll capture in your photograph.

But ISO works through amplification. Nothing changes inside your camera when you change your ISO setting: instead, the light on the sensor pixels are digitally amplified. If you have an amplifier in your house, you know what happens when you turn the nob up too high: you get considerable noise. The same thing happens with ISOs. You may capture an image of that rock band in concert at a darkly lit bar, but it’ll be a soup of static and noise.

In theory, the higher the ISO your camera supports, the better the camera will capture images in low-light environments. But since ISO is just digital amplification of the light being captured by pixels, a camera manufacturer can easily crank up a model’s ISO far past the point where the resulting images have an acceptable signal to noise ratio.

It’s hard to boil down what to look for in ISO ranges into a simple rule, but here’s two: if you care about how your camera works in low-light environments, read a review first or buy a DSLR. Barring that, follow this guideline: digital camera manufacturers exaggerate almost all cameras’ ISO ranges by about two logarithmic stops.

So if it says on the box that it goes up to 6400, then it’ll probably do 1600, (an ISO setting ideally capable of capturing most low-light environments just fine) and you’ll be getting a very decent camera.